This invention relates to hydrodenitrogenation of high nitrogen content hydrocarbon feeds in the presence of a multiple catalyst system.
Decreasing supplies of high quality petroleum crude oils have focused considerable attention on production and upgrading of lower quality petroleum crude oils as well as synthetic materials. Oil shale shows promise as an abundant as well as reliable source of hydrocarbons that can be converted to products of the type commonly obtained from petroleum hydrocarbons. Unfortunately, typical shale oils contain extremely high levels of nitrogen as well as significant amounts of oxygen as compared to many petroleum crude oils. Accordingly, to facilitate conversion of shale oils to useful products or products suitable for use as feed materials in conventional petroleum refining operations, treatment is required to reduce or remove nitrogen and oxygen.
Of course, nitrogen containing petroleum crude oils also are known and a number of processes for removal of nitrogen from nitrogen-containing feeds obtained from both petroleum and synthetic crude oils have been proposed. Among these are various solvent denitrification processes involving extraction of feeds with acids or polar solvents to remove nitrogen-containing molecules, as well as catalytic processes typically involving contacting a feed material with hydrogen in the presence of hydrodenitrogenation catalysts whereby nitrogen and hydrogen react to form easily removable nitrogen compounds such as ammonia without substantial destruction of hydrocarbon feed components with which the nitrogen was associated.
Typical catalysts employed in catalytic hydrodenitrogenation processes contain a hydrogenating metal component such as an oxide or sulfide of a Group VIB and/or VIII metal deposed on a refractory inorganic oxide support such as alumina. Examples of such catalysts are disclosed in U.S. Pat. No. 3,446,730 (Kerns et al.) and U.S. Pat. No. 3,749,664 (Mickelson).
Recently, workers in our laboratories have attained particularly good results in terms of hydrodenitrogenation of high nitrogen feeds such as whole shale oils and fractions thereof through the use of improved catalytic compositions comprising a chromium component, a molybdenum component and at least one Group VIII metal component deposed on a support component comprising a porous refractory inorganic oxide matrix component and a crystalline molecular sieve zeolite component. Such compositions and use thereof in hydrogen processing are disclosed and claimed in commonly assigned, copending application Ser. No. 200,536 of Tait et al. filed Oct. 24, 1980. Excellent results also have been attained using catalysts containing a similar hydrogenating component deposed on a support comprising silica and alumina according to commonly assigned, copending application Ser. No. 200,544 of Tait et al. filed Oct. 24, 1980, and with catalysts containing a hydrogenating component comprising a chromium component, at least one other Group VIB metal component and at least one Group VIII metal component and a phosphorus component deposed on a porous refractory inorganic oxide support as disclosed and claimed in commonly assigned, copending application Ser. No. 231,757 of Miller filed Feb. 5, 1981.
Although desirable results have been attained according to the above-described proposals, further improvements in hydrodenitrogenation of high nitrogen feeds would be desirable.
It is an object of this invention to provide an improved process for denitrogenation of high nitrogen content feeds. A further object is to provide an improved hydrodenitrogenation process wherein reactor throughputs are increased so that greater production of denitrogenated product is achieved for a given reactor volume. Another object of the invention is to achieve such results by a process which affords substantial savings in catalyst costs as compared to the aforesaid process in which the catalyst is a crystalline molecular sieve zeolite-containing catalyst. Other objects of the invention will be apparent to persons skilled in the art from the following description and the appended claims.
We have now found that the objects of this invention can be attained by hydrodenitrogenation of high nitrogen content feeds in the presence of a multiple catalyst system in which individual catalysts of the system are selected on the basis of reaction kinetics and rate constants to yield improved results in denitrogenation of high nitrogen feeds. While it is well known that the activity of various catalysts for hydrodenitrogenation reactions vary depending on catalytic composition, observed hydrodenitrogenation reaction kinetics of such catalysts in hydrodenitrogenation of hydrocarbon feed materials containing conventional levels of nitrogen are essentially first order following Langmiur-Hinshelwood kinetics given by the following equation: EQU R=K.sub.1 [N]/(1+K.sub.2 [N])
wherein R is the instantaneous hydrodenitrogenation reaction rate, K.sub.1 is the hydrodenitrogenation rate constant, [N] is instantaneous nitrogen concentration and K.sub.2 is the inhibition constant.
K.sub.2 is small for catalysts containing weakly-to-moderately acidic supports, e.g., alumina-supported catalysts. As a result, hydrodenitrogenation kinetics are observed to be first order with respect to nitrogen concentration. On the other hand, K.sub.2 unexpectedly has been found to be large for catalysts with more acidic supports, e.g., silica-alumina- or crystalline molecular sieve zeolite-alumina-supported catalysts. Accordingly, such catalysts are observed to exhibit less than first order kinetics, i.e., feed nitrogen exerts an appreciable inhibiting effect on reaction rate. The impact of the inhibition is especially significant at the high nitrogen concentrations typically found in shale oils and fractions thereof.
As observed for K.sub.2, the value of the rate constant, K.sub.1, has been found to vary with the acid strength of catalyst supports. K.sub.1 is determined from appropriate kinetic curves and equals the slope of the tangent to the curve near zero nitrogen concentration. For example, when [N] is near zero, K.sub.2 [N] also is very small. Accordingly, the instantaneous reaction rate, R, is essentially K.sub.1 [N]. At low nitrogen concentration, K.sub.1 can be determined in the usual way for first order reactions by plotting the log of product nitrogen concentration as a function of time and determining the slope. An important finding is that the rate constant, K.sub.1, is higher for catalysts having strongly acidic supports.
On the basis of these surprising findings, we have found that by using appropriate combinations of catalysts for hydrodenitrogenation, it is possible to obtain substantially improved hydrodenitrogenation rates as compared to those attained through the use of the individual catalysts. In fact, by appropriate selection of catalysts, hydrodenitrogenation rates up to 150% of those of the individual hydrodenitrogenation catalysts of the multiple catalyst system can be attained. In addition, as compared to the use of single catalyst systems in which the catalyst is a highly active one containing a crystalline molecular sieve zeolite component, appropriate combination of catalysts according to the present invention can yield not only improvements in denitrogenation, but also, savings in catalyst cost by virtue of reducing the amount of zeolite-containing catalyst employed.
In connection with the present invention it should be recognized that the use of multiple catalyst systems in refining operations is known. For example, U.S. Pat. No. 4,165,274 (Kwant) discloses a two-step hydrocracking process in which a tar sands oil distillate in first hydrotreated in the presence of a weakly or moderately acidic catalyst, such as a fluorine- and phosphorus-containing nickel-molybdenum on alumina catalyst, to reduce sulfur, nitrogen and polyaromatics content, after which the hydrotreated product is hydrocracked to a lower boiling product in the presence of a moderately or strongly acidic catalyst such as nickel-tungsten on low-sodium; type-Y molecular sieve. Similar two-step hydrocracking is conducted as part of a process for preparing medicinal oil and light hydrocarbon fractions such as naphtha and kerosene from heavy hydrocarbon oils such as vacuum distillates and deasphalted atmospheric and vacuum distillation residues according to U.S. Pat. No. 4,183,801 (Breuker et al.).
Although the above-described processes involve the use of multiple catalysts which may vary in acidity, the invented process differs in several respect. First, in the two-step hydrocracking process of Kwant and Breuker et al., each of the two steps has a distinct purpose, i.e., hydrotreating to remove contaminants in the first step and hydrocracking in the second step. In contrast, the process of the present invention makes use of a multiple catalyst system in which the predominant reactions throughout the entire system are hydrodenitrogenation. Hydrocracking may, though need not, accompany the denitrogenation. Neither Kwant nor Breuker et al. discloses or suggests a multiple catalyst bed process for hydrodenitrogenation nor do these patents address hydrodenitrogenation of high nitrogen content feeds such as are employed according to the present invention. Further, neither Kwant nor Breuker et al. suggests a process in which catalysts are manipulated on the basis of apparent reaction kinetics and rate constants for a single reaction, i.e., hydrodenitrogenation, to attain substantially improved results in terms of reactor throughputs.